This invention relates to a magnetic pickup and, in particular, to a magnetic pickup for sensing the speed of a rotor disc as utilized in a turbine or a compressor.
One prevalent method of sensing the speed of a rotating wheel in a rotary machine involves the use of a mask which is carried upon or affixed to the rotor wheel. The mask generally is formed of a nonmagnetic material and contains a series of permeable islands situated within the mask. The mask is passed through a stationary flux field established adjacent to the masked region whereby a voltage signal is generated as the permeable sites are passed therethrough. The voltage signals generated are then counted and converted to an output indicative of the rotor disc speed. Although this type of speed sensing device has proven to operate quite satisfactorily, its implementation has proven to be relatively expensive.
Magnetic speed pickup devices have also been developed which utilize a moving magnet, or series or magnets, which are carried or otherwise affixed to the rotating disc. The magnetic flux fields generated by these magnets extend outwardly from the wheel and are brought into communication with a transducer mounted adjacent the wheel. A signal is thus generated which is indicative of the rotor speed. In this arrangement, the relatively expensive mask is eliminated. Heretofore, the moving magnets have generally been secured to the rotating wheel by cementing the magnets in position using an epoxy resin, or the like. It has been found that with usage, the cement bond can fail, thereby allowing the magnets to fall into the moving machine components. This, in turn, can cause severe damage and/or produce an imbalance in the rotor system.